1. Field of the Invention
The present invention relates to a Doherty amplifier, and more particularly, to an apparatus for compensating for the phase of a Doherty amplifier.
2. Description of the Related Art
A Doherty amplifier is one of amplifiers used in a high efficiency modulation method for a high power transmitter and mainly improves efficiency by a combination of a B-class amplifier, a C-class amplifier, and an impedance inverting circuit.
FIG. 1 is a block diagram of a conventional Doherty amplifier. Referring to FIG. 1, the conventional Doherty amplifier includes a 3 dB hybrid coupler 110, a carrier amplifier 120, a peak amplifier 130, and a quarter wave transformer 140 having a second transformer 142 and a first transformer 141 coupled with output port 150 as shown.
The Doherty amplifier adopts a method of connecting the carrier amplifier 120, and the peak amplifier 130 in parallel using the quarter wave transformer 140 (λ/4 lines). The amount of current output from the peak amplifier 130 as a load varies according to a power level. Accordingly, efficiency is improved by adjusting a load impedance of the carrier amplifier 120.
In the 3 dB hybrid coupler 110, a splitter splits a signal into two signals so that one signal is input to the carrier amplifier 120 and the other signal is input to the peak amplifier 130. The signal input to the peak amplifier 130 is delayed by 90° so that a delay time difference with the signal input to the carrier amplifier 120 is compensated for.
Although the 90° phase compensation is theoretically possible, in an actual circuit, since the delayed time is not exactly 90° due to various constituent components existing in the circuit, a phase equivalent to the actual difference is to be compensated. The 3 dB hybrid coupler 110 is mainly embodied using passive devices.
The carrier amplifier 120 and the peak amplifier 130 are formed in two stages or three stages and include input matching, a drive port transistor, inter-stage matching, an output port transistor, and an output matching network.
However, the 3 dB hybrid coupler 110 formed of the passive devices requires the passive devices having large sizes to be embodied at a lower frequency so that miniaturization thereof is difficult.